1 degenerate stars there is not a sharp transition between relativistically degenerate and non-...
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1 Degenerate stars There is not a sharp transition between relativistically degenerate and non- relativistically degenerate gas. Similarly there is no sharp transition between an ideal gas and a completely degenerate one. Partial degeneracy situation requires much more complex solution. Isothermal degenerate C/O core Thin non- degenerate surface layer of H or He White dwarfs Intrinsically faint, hot stars. Typical observed masses 0.1-1.4M Calculate typical radius and density of a white dwarf ( =5.67x10 -8 Wm -2 K -4 ) Slide 2 2 Example of WD discovered in Globular cluster M4 Cluster age ~ 13Myrs WDs represent cooling sequence Similar intrinsic brightness as main-sequence members, but much hotter (hence bluer) Slide 3 3 Measured WD masses Mass estimates for 129 white dwarfs From Bergeron et al. 1992, ApJ N Mean M = 0.56 0.14 M How is mass determined ? Note sharp peak, and lack of high mass objects. Slide 4 4 Observed mass-radius relation Mass/radius relation and initial mass vs. final mass estimate for WD in stellar clusters. How would you estimate the initial mass of the progenitor star of a WD ? Koester & Reimers 1996, A&A, 313, 810 White dwarfs in open clusters (NGC2516) Slide 5 5 Relativistic treatment of the equation of state imposes upper limit on NS mass. Above this mass, degeneracy pressure unable to balance self-gravity. Complications: General Theory of Relativity required Interactions between neutrons (strong force) important Structure and maximum mass equations too complex for this course Outer Crust: Fe and n-rich nuclei, relativistic degenerate e Inner Crust: n-rich nuclei, relativistic degenerate e Interior: superfluid neutrons Core: unknown, pions ?quarks ? Various calculations predict M max =1.5 3M solar Slide 6 6 Discovery of neutron stars 1967: Hewish and Bell discovered regularly spaced radio pulses P=1.337s, repeating from same point in sky. Approx. 1500 pulsars now known, with periods on range 0.002 < P < 4.3 s Crab pulsar - embedded in Crab nebula, which is remnant of supernova historically recorded in 1054AD Crab pulsar emits X-ray, optical, radio pulses P=0.033s Spectrum is power law from hard X-rays to the IR Synchrotron radiation: relativistic electrons spiralling around magnetic field lines. Slide 7 7 Pulsar mechanism Rapidly rotating NS with strong dipole magnetic field. Magnetic field axis is not aligned with rotational axis. Spectrum of Crab pulsar is non- thermal. Suggestive of synchrotron radiation - relativistic charged particles emit radiation dependent on particle energy. Charged particles (e-) accelerated along magnetic field lines, radiation is beamed in the the acceleration direction. If axes are not aligned, leads to the lighthouse effect Slide 8 8 Black Holes Description of a black hole is entirely based on theory of General Relativity - beyond scope of this course. But simple arguments can be illustrative: Black holes are completely collapsed objects - radius of the star becomes so small that the escape velocity approaches the speed of light: Escape velocity for particle from an object of mass M and radius R If photons cannot escape, then v esc >c. Schwarzschild radius is Slide 9 9 ObjectM (M )RsRs Star1030 km Star39 km Sun13 km Earth3x10 -6 9 mm Size of black holes determined by mass. Example Schwarzschild radius for various masses given by: The event horizon is located at R s - everything within the event horizon is lost. The event horizon hides the singularity from the outside Universe. Two more practical questions: What could collapse to from a black hole ? How can we detect them and measure their masses ? Slide 10 10 How to determine compact object masses P = orbital period K c = semiamplitude of companion star i = inclination of the orbit to the line of sight (90 o for orbit seen edge on) M BH and M c = masses of invisible object and companion star Keplers Laws give: The LHS is measured from observations, and is called the mass function f(m). f(m) 0 Hence we have firm lower limit on BH mass from relatively simple measurements